By 1914, G Scammell & Nephew Ltd, now run by brothers Alfred George and Allan Howard Scammell had filled their Fashion Street, Spitalfields, London, workshops to capacity, had become agents for Commercial Cars and owned six Foden wagons which were available for hire. Further workshops had been added to the original building and a floor built above them for office accommodation. Under the leadership of the Scammell brothers, G. Scammell & Nephew had taken full advantage of the growing motor age and close proximity to the busy docklands to become a flourishing and ever expanding business.

Inevitably, the onset of the Great War had a dramatic impact on the company. Alfred, who had served with the 13^th Middlesex Regiment Volunteers from 1902, before transferring to the 5^th London Brigade Royal Field Artillery TF in 1908 rejoined his regiment having been promoted to Major in 1912. He was sent to France where he served with Royal Field Artillery, 235 Brigade, 47 Division.

On 25 January 1916 he was injured at Loos when a shell exploded close to him and rendered him deaf. As a result of this he left his unit on 5 July 1916 and returned to England, via Boulogne, on 7 July. His medical report stated that permanent deafness would result if he was again subjected to the noise of shellfire in close proximity. He was therefore transferred to Home Front duties. He was promoted to Lieutenant-Colonel on 9 March 1917 and assumed command of "C" Battery, 6 Reserve Brigade RFA at Biscot Camp, Luton on 11 August 1917.

On 14 September 1918, Lt.Col AG Scammell wrote to the Brigade Commander, 6 Reserve Brigade requesting an extended period of leave for six months. The reason he gave was that his brother, Allan, was seriously ill and unable to continue the running of G Scammell & Nephew. He sought to strengthen his request by stating that the company was involved with contracts with the Royal Navy and the Royal Air Force valued at £250,000 (£14,527,640 in 2020). The War Office appeared not too keen on him taking six months leave and invited him to resign his commission. This he did on 19 February 1919, after receiving assurance that he could keep his rank and be entitled to wear his uniform and he duly returned to the helm at G Scammell & Nephew.

During the war, with AG Scammell absent with the military, the running of G Scammell & Nephew was the responsibility of AH Scammell, whom the War Office considered more valuable in that role than in the military. One of the first big jobs for the War Office was for the supply of Foden steam wagons in mid to late September 1914. The Commercial Motor records that the company was asked by the War Office to provide twenty-eight fully fitted Fodens, six trailers and personnel within 3.5 days, and praises the company for finding the wagons and completing the task in the available time.

While it is likely that the company provided the men to service and maintain the column, it is unlikely that the company were instrumental in supplying the wagons and drivers. These would have been requisitioned by the War Office and as such delivered complete with crew. Steam wagons were only requisitioned if accompanied by sufficient crew to operate them. Requisitioned petrol vehicles were always taken with volunteer drivers and those that were included in the Subsidy Scheme only qualified for the scheme if the driver had enlisted into the Territorial Force. Steam wagons were only requisitioned if accompanied by sufficient crew to drive them. It was nevertheless a fantastic effort to inspect, repair and paint twenty-eight wagons within the time frame and stood the company in good stead with the War Office for future work.

In 1916 G Scammell & Nephew came under the control of the Ministry of Munitions and the workload of the company would have been for the most part war work. This would have included work for Home Front Army Service Corps companies in addition to contracts for the Royal Navy and Royal Flying Corps. Only civilian work of the utmost urgency would have been approved by the Ministry of Munitions. The difficulties of managing an increasing workload with a diminishing staff of skilled men can only be imagined as the workshops were kept busy day and night, seven days a week.

Although war work was always the priority, civilian work became increasingly important. No new vehicles were available, but there was an increasing need for transport to and from the docks and the increasing number of factories employed on war work. Civilian life also continued with the population needing to earn a living. Severe strain was put on older vehicles, many overdue for replacement which had to be kept running. The business was kept running day and night every day of the week and by 1917 had expanded to occupy every building on both sides of the road for half the length of Fashion Street. Even so, the pressure on space was such that the local authorities relaxed peace-time planning restrictions to permit the storage of vehicles in the street, effectively turning Fashion Street itself into an extension of the works.

The strain of keeping the company running week in week out finally took its toll on Allan Scammell, who suffered a haemorrhage of the lungs in September 1918. He was advised by his doctors to retire to Bournemouth for the cleaner air and not to return to London. Lt Col AG Scammell returned to G. Scammell & Nephew to take over the reins after resigning his army commission.

In 1922, production of the new Scammell lorry was moved to new premises in Tolpits Lane, Watford, Hertfordshire. Scammell Lorries Ltd was formed as a separate entity to G Scammell & Nephew which was subsequently sold to Henry Hood Barrs ending the Scammell association with the company after some 85 years.